A Beginner's Guide to Black Hole Imaging and Associated Tests of General Relativity

Following the 2019 release by the Event Horizon Telescope Collaboration of the first pictures of a supermassive black hole, there has been an explosion of interest in black hole images, their theoretical interpretation, and their potential use in tests of general relativity. The literature on the subject has now become so vast that an introductory guide for newcomers would appear welcome. Here, we aim to provide an accessible entry point to this growing field, with a particular focus on the black hole "photon ring": the bright, narrow ring of light that dominates images of a black hole and belongs to the black hole itself, rather than to its surrounding plasma. Far from an exhaustive review, this beginner's guide offers a pedagogical review of the key basic concepts and a brief summary of some results at the research frontier.